This invention relates to an energy converter, whereby a load is supplied with energy from a source, utilising a series-resonant bridge circuit containing at least two thyristors with diodes connected antiparallel thereto and a high-frequency output transformer, and an energy buffer. Furthermore, a control circuit is present for generating trigger pulses in response to the energy supplied to and extracted from the buffer to bring the appropriate thyristors into the conducting state, which control circuit includes diode current detectors for supplying signals indicative of the presence of current flowing through said diodes. Switching means are provided for passing the trigger pulses to the thyristors in the bridge circuit, conditional upon the presence of current flowing through the respective diodes antiparallel connected to the thyristors conducting last and directly before said diodes.
Such energy converters have been known for a long time and their purpose is to convert the energy from a single phase or a polyphase AC source or from a DC source into energy with a single phase or polyphase AC voltage of a different amplitude and/or frequency, or into a pulsating voltage or with a DC voltage. With the relevant thyristors of the bridge circuit in the conducting state, energy from the source is supplied to an energy buffer via the series resonant circuit, and energy is extracted from the buffer by the load. During the time when the thyristors are in the conducting state the energy supplied to the buffer may be returned to the source via the series-resonant circuit or extracted by the load. The energy balance setting is determined by the times at which the thyristors of the bridge circuit are triggered.
An energy converter as set forth in the opening paragraph is disclosed, for example, in the "IEEE Transactions on Industrial Electronics and Control Instrumentation", vol. IECI-23, No. 2, May 1976, pp. 142-150. The energy converter described in this article comprises a control circuit for delivering, in response to the current flowing in the series-resonant bridge circuit and to a voltage derived from the output voltage of the output transformer, trigger pulses to bring the appropriate thyristors into the conducting state. It may however occur that a thyristor of the bridge circuit remains in the conducting state for a longer time than intended, while a thyristor connected in series therewith should already have been brought into the conducting state. Should the latter occur, this means a short-circuit in the bridge circuit. To prevent such short-circuits, a rather long time would have to elapse before the latter thyristor (i.e. the off thyristor brought into the conducting state. As a consequence, a large ripple would arise on the output signal of the energy converter.
According to the European patent specification No. 0 071 285, such a disadvantage is solved with a full bridge circuit as the energy converter contains two antiparallel-connected thyristors in series with the bridge circuit. These thyristors prevent the situation that, during the resting period occurring when the energy converter operates in the trigger mode, no current is flowing in the bridge circuit. This ensures in the first instance that a capacitor in the series-resonant circuit, after the supply of energy to the load, is not further discharged. As a result of this, the diode connected antiparallel to the thyristor last conducting starts to draw current immediately after this thyristor is blocked.
The above solution has the disadvantage that two additional control circuits re required for driving and extinguishing the two antiparallel-connected thyristors. Besides, these two thyristors should be of the symmetric type because the voltage across the thyristors, with the switching of the bridge circuit, changes in polarity while the amplitude remains the same. Consequently, there is a relatively high voltage across the two antiparallel-connected thyristors if one of these is in the conducting state, causing the efficiency of the bridge circuit to be decreased. Moreover, through the loss in efficiency the bridge circuit, on account of the greater heat development, has to be accommodated in a larger space. Because of the current flowing in the series-resonant circuit during the so called resting period of this circuit, it may happen that the thyristors fail to extinguish at the correct moment. To prevent this, additional measures have to be taken, as described in the cited European patent specification No. 0 071 285.